UK Pat. No. 1320441 is concerned with drafting apparatus for measuring the co-ordinates of points and additionally for making a drawing in accordance with co-ordinates. It particularly teaches that the position of a drafting head can be read from separate X and Y scales as X and Y co-ordinates from a given zero co-ordinate. The X scale is mounted along the top edge of the drafting apparatus whilst the Y scale is mounted along a Y-bar or beam extending at right angles to the X scale. The draftsman is accordingly compelled to move to two separate positions to read the X and the Y co-ordinates of each point and the accuracy of each reading will consequently depend on the draftsman's personal skill in using the respective scale reading devices consistently.
Although the X and the Y co-ordinates are provided from a given zero co-ordinate, this is completely arbitrary and there is no provision for adjusting this zero co-ordinate to suit, for instance, a particular drawing, or to assist with the computation of dimensions on the drawing.
UK Pat. No. 1327658 is concerned with similar drafting apparatus and more particularly with the problem of conversion between metric and imperial systems of measurement. This patent specification teaches that this is achieved by providing dual X and Y scales and consequently fails to resolve the problems inherently associated with apparatus in accordance with UK Pat. No. 1320441.